Heat-treating food products for increased shelf life is a well-known and often-employed method. For example, the food product may be various dairy produce such as milk, cream or yoghurt. The heat treatment may take place in a plurality of ways, either directly or indirectly. Indirect methods are, for instance, heating by means of different types of heat exchangers. Of the direct methods, there are two major groups, injection of infusion with steam. By employing a direct method, an extremely rapid heating will be obtained which today is to be sought after since, in order to improve the flavour properties of, for example, milk, it is often the intention to heat to elevated temperatures for brief periods of time.
The present invention relates to an apparatus which employs infusion heating. Infusion entails that a finely-divided liquid is heated in a steam chamber. The principle of heating a liquid, for example a liquid food, by injecting the food into a chamber filled with steam has been known since the early part of the nineteenth century.
The first infusion heating plants were based on the same principle as condensers, by means of which excess steam from various chemical processes was condensed. The earlier plants thus displayed an infusor which consisted of an autoclave with an inlet for the product in the upper region of the autoclave. The inlet discharged in a distributor chamber which divided the incoming product into small, fine liquid droplets. At the bottom of the autoclave, there is an outlet for the heated product, The autoclave was further provided with an inlet for steam which, in these early infusors, was located in the lower region of the autoclave. A drawback inherent in these infusors for, for example, milk is however that the steam which is injected at a low level into the autoclave towards the finely-divided liquid droplets causes the droplets to change direction and many of the droplets became stuck to the hot inside walls of the infusor where burning of the product was the immediate outcome. Burning of the product onto such hot walls entails major hygienic problems and such burnt product is extremely difficult to wash off.
One method of obviating the problem with burning of the product onto the inner walls of the infusor is to place the steam inlet in the upper region of the infusor. The steam is fed from above over the distributor chamber of the product so that the droplets are not disrupted in their fall down through the steam chamber. But the admission of steam concurrently with the admission of product also has its drawbacks. This type of steam admission may give rise to burning of product around or in the distributor apertures of distributor gaps displayed by the distributor. This disrupts the heating of the product, the temperature of the product falls, and so the steam pressure must be increased and higher temperature differences will occur between product and steam. By designing the distributor apertures such that they will have a very thin edge out towards the steam chamber, this problem may be solved in that very thin layers of product will be burnt fast which are easily broken off before becoming disruptive.
However, the concurrent admission of steam also entails problems with the uncondensable gases which are always to be found in the product and which are emitted in connection with its heating. The steam also contains a quantity of these gases. The gases may consist of oxygen, nitrogen, carbon dioxide and others. These gases must be continuously removed in order for it to be possible to maintain the steam temperature at the same pressure. As a result of a concurrent steam admission, the uncondensable gases more readily mix with the steam and it becomes difficult to remove them without also unnecessarily releasing steam out from the process.
It would, thus, entail major advantages if it were possible to employ the "classic" solution with a steam inlet placed well down on the infusor without suffering from the problems involved in burning of product from which this solution previously suffered.